Synchronizing means



Dec. 13,1938. D. c ESPLEY 2,14 0

' I SYNCHRONIZING MEANS Filed Aug. 10, 1937 FIG.3

3 11 1o 1 2 5/; Egg/1 13 FIG. 4

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Patented Dec. 13, 1938 PATENT OFFICE SYNCHRONIZING MEAN S Dennis Clark Espley, North Wembley, England,

assignor to The General Electric Company Limited, London, England Application August 10, 1937, Serial No. 158,377 In Great Britain August 11, 1936 Claims.

This invention relates to apparatus for transmitting television and the like of the type wherein electrical synchronizing signals are generated by the co-operation of holes in a rotating discwith 5 a slit or slits relative to which the disc rotates. The two terms hole and slit are used merely to distinguish apertures in the disc from apertures in the body relative to which the disc rotates; nothing is implied by them about the shape of the holes; in particular it is not implied that the shapes of the holes and slits are necessarily different. In such means the conversion of the relative motion of hole and slit into electrical signals is, of course, usually effected by a beam of light, passing from a source to a photoelectric cell, through a hole and slit.

In means of this type it is desirable that, when the sequence of synchronizing signals is analyzed into a Fourier series, the lowest or fundamental frequency should be that corresponding to the rate at which the holes pass the slit. Thus, if there are N holes round the disc which rotates n times per second, (e. g. N=120, 12 50), the lowest frequency should be Nn cycles per second. Multiples of this frequency are not objectionable; indeed they may be necessary if the signals are to have a square wave-form; but frequencies of the form gm or N11,: p n, where p is much less than N, are objectionable.

But these objectionable frequencies will appear unless all the holes are accurately of the same size and are all spaced at exactly equal intervals. If a single hole is of a different size from the rest or differently spaced from its neighbours, a frequency n and a frequency N'rtin will appear in the Fourier series. It has been proposed to eliminate these unwanted frequencies by filters; but if N is large, a filter so'selective that it will eliminate Nni'n. is diflicult to obtain. The object of the invention is to prevent the appearance of such frequencies, so that there is no need for their subsequent elimination.

According to the invention this object is attained by superimposing on the generation of the signals a positive irregularity which substantially counteracts an irregularity in the size and/or spacing of the holes. The term positive implies that the means for producing the irregularity must be such that the nature and amount of the irregularity they produce must be independent of the nature and amount of the irregularity associated with the holes. (This statement will be qualified in one respect later.) They are thus distinguished from the filters aforesaid, which produce no effect on the signals unless the frequencies they are designed to eliminate are actually present.

A very simple illustration will serve to explain further the nature of the invention.

Suppose that the only irregularity in the holes consists of the displacement of one hole by a distance a: from its proper position, so that its signal is generated slightly too early. Then, according to the invention, there might be operated by the rotating disc means which, when this hole Q. was approaching the slit, displaced the slit by the distance r, so that the hole passed the slit at its proper time. Alternatively, but of course less easily, a delay circuit of. known .kind might be switched into the path of the signals when the;.15 abnormal hole was approaching, and switched out again after it has passed; this delay circuit would again cause the signal due to the abnormal hole to pass down the path at the correct time.

This example is given chiefly in order to indi- 20 cate that the means producing the compensating irregularity may be either mechanical or electrical, and may reside at the slit or in the circuit. In considering what compensating irregularities will be practicable and effective, it must ggg be remembered, first that the irregularities to be compensated are small, so that a compensation which is theoretically very imperfect may. actually be satisfactory; second that the irregularities that have to be compensated are those for go which the integer p is not much greater than 1; e. g. if N=l50, those for which p is less than 10; irregularities for which p is greater can be'eliminated by filters. Accordingly any irregularity can be adequately compensated by the super-. 35 position of counteracting irregularities of a few frequencies 1m, their amplitude and phase being suitably chosen. Moreover since the irregularities are all small, it is not necessary to inquire too closely into their nature. In general, irregularities of size (or spacing) of hole should be compensated by an irregularity of size (or spacing), of slit, or by its equivalent in the circuit. But sometimes it will be found sufiicient to compensate an irregularity of size by an irregularity of spacing, or vice versa.

Several practicable methods of producing the compensating irregularities will now be described with reference to the accompanying drawing, in which each figure illustrates (but without constructional detail) one method of production.

Figures 1 and 2 illustrate two forms of the synchronizing disc with an inclined slit.

Figure 3 illustrates an adjustable fixed slit.

Figures 4 and 5 illustrate two forms of vibrating slits.

At the outset it is necessary to distinguish between synchronizing holes arranged in a circle and those arranged on a spiral. If the synchronizing holes are arranged on a spiral, the compensating irregularity can generally be a fixed slit of appropriate form and position; if the irregularity to be compensated is one that increases regularly fromone end of the spiral to the other, very simple means will suffice to compensate it. Thus, as shown in Figure 1, a fixed slit I with parallel jaws may be inclined at a substantial angle to the radius 2 of the disc 3. If the rotation is in the direction indicated by the arrow, each hole 4 will arrive at the slit a little earlier or later than it would have done if the slit had been radial; accordingly an irregularity consisting of a spacing that increases regularly along the spiral can be compensated.

If the size (and not the spacing of the holes) increases regularly along the spiral, the arrangement of Figure 2 may be more appropriate. Here the jaws of the slit 2 are inclined to one another, so that the outer smaller holes are exposed to the light longer than the inner larger holes.

A rather more complicated method, adapted to correct an irregularity of frequency pin and another of frequency pan is shown in Figure 3. In the figure p1=1 and 192:2; but it will be evident how the device may be extended to other values. Here one jaw 5 of the slit has n projections 6, each extending for a length e/2pi, separated by equal gaps. (e is the overall radial length of the spiral). The other jaw J has 102 projections 8, each extending for a length 6/2172, separated by equal gaps. Each jaw is movable parallel and perpendicularly to the length of the slit. When the jaws are arranged as shown, the effective slit has the shape shown by the shaded area 9. By moving the jaws and adjusting this shape, the irregularity produced, which will have main components of frequency pm and pm, can be adjusted in amplitude and phase so as to compensate the said irregularities of these frequencies.

If it is necessary to correct for more than two frequencies, the jaws in Figure 3 may be of more complicated shape. Then it is best to determine their shape by trial from the observed scan distortion. But final adjustment may be still by relative motion of the jaws.

Alternatively the effect of the slit in Figure 2 or that of the simpler forms of the slit in Figure 3 could be simulated by the introduction of absorbing matter into the path of the light passing through those holes that are too large. The amount of light reaching the cell through each of the holes can thus be made substantially equal by an irregularity, not in time, but in space. Many methods of introducing such absorbing matter will be evident tothose skilled in the art.

But if the synchronizing holes are on a circle, and the compensating irregularity is to be produced at the slit, a motion of the slit, real or virtual, is necessary. Figures 4 and 5 show methods of this kind. The holes are now shown on a circle; but of course these methods may also be used with holes arranged on a spiral.

In Figure 4 the slit I is supported by an elastic strip I0, held at one end by the block II and carrying at the other the armature l2 cooperating with the electromagnet I3. This electromagnet is fed with current of the general form i=2nAn cos (2npnt+6n) and thus the slit made to vibrate with any desired motion compounded of frequencies pn with any amplitudes and phases; corresponding irregularities are introduced into the signals.

The arrangement of Figure 5 diiTers from that of Figure 4 in that the width rather than the position of the slit is varied by the electromagnet l3. The two jaws l4, 14 are each supported on an elastic strip [3, It) held in the block H and vibrated in the manner of a tuning-fork by the action of the magnet on the armatures l2, 12'.

The vibration of the slit of Fig. 4 or the slit jaws of Fig. 5 may be effected in proper synchronism by energizing the magnet l3 with A. 0. current of the required frequency derived from the current operating a synchronous motor rotating the scanning disc. For general instructions as to obtaining the proper frequencies and the separation of harmonics for this purpose, reference may be had to Sheas transmission networks and wave filters.

However, all that is required here is to obtain the required frequencies from the signals themselves, for the signals contain frequencies Nn+pn and it is the frequencies 2m that are required. The problem of separating various frequencies has been solved for the ordinary radio receiving set and the same method will apply here, so reference may be had to any text book on wireless receiving.

By whatever means the compensating irregularity is produced, its frequency or frequencies will have to be pn. Since the disc is generally driven by a synchronous motor, the most convenient plan will usually be to derive the frequencies from the driving supply. But any alternative method may be: used. Thus if the compensating means are mechanical, they may be driven mechanically from the disc. However, one method deserves special notice. If the irregularity to be compensated is compounded of several frequencies pn, these frequencies may be separated in known manner from the signals in an earlier stage of their path and caused to control the compensating irregularity introduced into a later stage of the path. This plan is most feasible when the compensating irregularity is introduced electrically; it represents the one exception aforesaid to the condition that the nature of the compensating irregularity must be independent of the nature and amount of the irregularity associated with the holes.

Hitherto it has been tacitly assumed that the original irregularities were accidental. It is'possible that they may be intentional. Thus the picture frequency synchronizing signals are sometimes generated by omitting a few holes that generate the line frequency synchronizing signals. When this procedure is adopted, the intentional irregularities may still be counteracted according to the invention; but the counteraction must clearly take place after the line frequency signals have been separated from the picture frequency signals. The counteracting irregularity will then be electrical and take place at some point in thepath after the separation.

I claim:

1. Synchronizing means for television comprising a uniformly rotatable scanning disc provided with a series of synchronizing holes which normally should be uniform in size and spacing but which involve inherent irregularities and means including a slit arranged to cooperate with said synchronizing holes for compensating for said irregularities so that the light for the synchroniz- 4. Synchronizing means for television as in claim 1 wherein the synchronizing holes are arranged in a circle and means is provided for vibrating at least one of the jaws of the cooperating slit for effecting the required compensation. 5

5. Synchronizing means for television as in claim 1 wherein the synchronizing holes are arranged in a circle and means is provided for vibrating the jaws of the cooperating slit transversely of a radius of the disc for effecting the 10 required compensation.

DENNIS CLARK ESPLEY. 

